Worst mistake to ever make in the Pokémon world...

[Sceptile987]

punching azelf in the face

buying the slowpoketail for 1000000
annoying a salamence

Edit by GL - Please don't double post, in future just edit the post if you wish to add something else onto your previous point =]

[Kdintranet & Scizau]

Catching a level 100 Magikarp in the resort area on Platinum.

[Utack and Swampy]

Picking up item balls inside the powerplant

[RioluRiku]

Picking Chimchar. Also, not picking Torchic is equally as bad, if not worse.

I did both of those.
and never regreted it.

1. Fall for your pokemons attract move.
2. asking jirachi for money.
3. try to catch the ghost in lavender town
4. fight machamp in a arm bar match
5. use rocks on spearows without having any pokemon

[Kdintranet & Scizau]

Not getting the Bike
Not catching a pidgey in R/B/Y
Forgetting to do Sabrina till the end.

[lugia95]

Wasting your masterball on a trainers pokemon (my friends brother did this on Sabrina's Alakazam)

[LaserDuck]

DO NOT under ANY circumstances, EVER, aim for the horn.....

and letting arbok evolve to seviper is a bit of a fail too.....

[Gochiruzeru]

Hatching an egg without a bike.

Seriously, it's horrible. X_x

[Kdintranet & Scizau]

Climbing to the top of Mt Silver without a Pokemon higher than level 50. Its horrible.

[HipHop Honchkrow]

Have a pikachu at the side of your shoulder... 

[lugia95]

Saying 'yes' if anyone apart from a gym leader asks for your pokegear number. They dont leave you alone.

[Catstorm]

Trashing your Escape Ropes and going right into Mt. Silver. D:

[Kdintranet & Scizau]

Picking Hitmonchan in Pokemon Fire Red.

[aer_dingus]

trying to punch Mewtwo. nice try Ash, you idiot, like that is going to do anything.

[Ninetalesgirl421]

Actually BUY a bike
Try to fight a gym leader with weak, type-disadvantaged pokemon
Train only Smeargles